linux-kernel - Re: x86's nmi_hz wrt. oprofile's nmi_timer

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Date:	Sun, 22 Feb 2009 20:11:52 -0800 (PST)
From:	David Miller <davem@...emloft.net>
To:	andi@...stfloor.org
Cc:	linux-kernel@...r.kernel.org
Subject: Re: x86's nmi_hz wrt. oprofile's nmi_timer_int.c

From: Andi Kleen <andi@...stfloor.org>
Date: Sun, 22 Feb 2009 18:06:45 +0100

> David Miller <davem@...emloft.net> writes:
> 
> Really old mail, but I was very behind. I didn't see an 
> correct answer, so let's answer it.
> 
> > While working on an NMI watchdog implementation on sparc64
> > I noticed what seems to be a peculiar behavior of the NMI
> > timer int oprofile support on x86.
> >
> > When the NMI watchdog tests itself at boot timer we start
> > with nmi_hz equal to HZ.
> >
> > After the NMI watchdog self-test passes, nmi_hz is reduced
> > down to '1'.
> >
> > The NMI timer int oprofile support simply uses DIE_NMI notifiers for
> > it's implementation.  But I don't see anything in the code of
> > arch/x86/oprofile/nmi_timer_int.c nor the NMI watchdog infrastructure
> > which will re-adjust nmi_hz back to HZ or something similar.
> >
> > Am I missing something?
> 
> oprofile generates its own NMIs, it does not rely on 
> the ones from the nmi watchdog.

The code in nmi_timer_int.c doesn't.

> In timer mode it does not use nmis or die notifiers, but relies on the 
> regular non nmi timer interrupt.

Again, the code in nmi_timer_int.c doesn't.

It uses the NMI watchdog timer interrupts, it catches DIE_NMI
events.

> Does that answer your question?

Not really.
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